1. Field of the Invention
The present invention relates to a cap of a fuel tank with a pressure control valve for controlling pressure in the fuel tank.
2. Description of the Related Art
One known example of fuel tank caps is disclosed in JAPANESE UTILITY PATENT PUBLICATION GAZETTE No. 6-88606. FIG. 38 is a sectional view illustrating a fuel tank cap 300. As shown in FIG. 38, the fuel tank cap 300 includes a plastic casing body 302 screwed to a filler neck FN of a fuel tank (not shown), a cover member 330 attached to the casing body 302, and a negative pressure valve 340 received in a valve chamber 304 of the casing body 302 for controlling pressure in the fuel tank. The negative pressure valve 340 includes a rubber valve body 342, a valve supporting member 346 with a fitting aperture 346a for supporting the valve body 342, and a spring 348 for pressing the valve body 342. When the differential pressure between the tank pressure and the atmospheric pressure applied to the valve body 342 increases to or above a predetermined level, the negative pressure valve 340 opens to make the tank pressure approach atmospheric pressure.
The casing body 302 has threads 302a formed on the circumference thereof to be screwed to the filler neck FN. The casing body 302 is thus required to have high mechanical strength. The casing body 302 has a valve chamber 304 in which the negative pressure valve 340 is received. High dimensional accuracy is required to enhance the sealing characteristics of the fuel cap. If the wall thickness of the casing body were increased to enhance the mechanical strength of the casing body 302, then sink marks are formed in injection molding which deteriorate the dimensional accuracy.